Choose an authentication mode
Applies to: 
SQL Server
During setup, you must select an authentication mode for the Database Engine. There are two possible modes: Windows Authentication mode and mixed mode. Windows Authentication mode enables Windows Authentication and disables SQL Server Authentication. Mixed mode enables both Windows Authentication and SQL Server Authentication. Windows Authentication is always available and can't be disabled.
Configuring the authentication mode
If you select Mixed Mode Authentication (SQL Server and Windows Authentication mode) during setup, you must provide and then confirm a strong password for the built-in SQL Server system administrator account named sa. The sa account connects by using SQL Server Authentication.
If you select Windows Authentication during setup, the setup creates the sa account for SQL Server authentication but it's disabled. If you later change to Mixed Mode Authentication and you want to use the sa account, you must enable the account. Any Windows or SQL Server account can be configured as a system administrator. Because the sa account is well known and often targeted by malicious users, don't enable the sa account unless your application requires it. Never set a blank or weak password for the sa account. To change from Windows Authentication mode to Mixed Mode Authentication and use SQL Server Authentication, see Change Server Authentication Mode.
Connecting through Windows Authentication
When a user connects through a Windows user account, SQL Server validates the account name and password using the Windows principal token in the operating system. This means that the user identity is confirmed by Windows. SQL Server doesn't ask for the password, and doesn't perform the identity validation. Windows Authentication is the default authentication mode, and is much more secure than SQL Server Authentication. Windows Authentication uses New Technology LAN Manager (NTLM) or Kerberos security protocol, provides password policy enforcement with regard to complexity validation for strong passwords, provides support for account lockout, and supports password expiration. A connection made using Windows Authentication is sometimes called a trusted connection, because SQL Server trusts the credentials provided by Windows.
For information on configuring Kerberos, see Register a Service Principal Name for Kerberos connections.
By using Windows Authentication, Windows groups can be created at the domain level, and a login can be created on SQL Server for the entire group. Managing access at the domain level can simplify account administration.
Important
When possible, use Windows authentication.
Connecting through SQL Server Authentication
When using SQL Server Authentication, logins are created in SQL Server that aren't based on Windows user accounts. Both the user name and the password are created by using SQL Server and stored in SQL Server. Users connecting using SQL Server Authentication must provide their credentials (login and password) every time that they connect. When using SQL Server Authentication, you must set strong passwords for all SQL Server accounts. For strong password guidelines, see Strong Passwords.
Three optional password policies are available for SQL Server logins.
User must change password at next login
Requires the user to change the password the next time that the user connects. The ability to change the password is provided by SQL Server Management Studio. Other company software developers should provide this feature if this option is used.
Enforce password expiration
The maximum password age policy of the computer is enforced for SQL Server logins.
Enforce password policy
The Windows password policies of the computer are enforced for SQL Server logins. This includes password length and complexity. This functionality depends on the
NetValidatePasswordPolicyAPI, which is only available in Windows Server 2003 and later versions.
To determine the password policies of the local computer
On the Start menu, select Run.
In the Run dialog box, type secpol.msc, and then select OK.
In the Local Security Settings application, expand Security Settings, expand Account Policies, and then select Password Policy.
The password policies are described in the results pane.
Disadvantages of SQL Server Authentication
If a user is a Windows domain user who has a login and password for Windows, they must still provide another (SQL Server) login and password to connect. Keeping track of multiple names and passwords is difficult for many users. Having to provide SQL Server credentials every time the user connects to the database can be annoying.
SQL Server Authentication can't use Kerberos security protocol.
Windows offers additional password policies that aren't available for SQL Server logins.
The encrypted SQL Server Authentication login password must be passed over the network at the time of the connection. Some applications that connect automatically will store the password at the client. These are additional attack points.
Advantages of SQL Server Authentication
Allows SQL Server to support older applications and applications provided by third parties that require SQL Server Authentication.
Allows SQL Server to support environments with mixed operating systems, where all users aren't authenticated by a Windows domain.
Allows users to connect from unknown or untrusted domains. For instance, an application where established customers connect with assigned SQL Server logins to receive the status of their orders.
Allows SQL Server to support web-based applications where users create their own identities.
Allows software developers to distribute their applications by using a complex permission hierarchy based on known, preset SQL Server logins.
Note
Using SQL Server Authentication does not limit the permissions of local administrators on the computer where SQL Server is installed.